We investigated the orientation and morphology of Cu nanoparticles grown under ultrahigh-vacuum conditions on ZnO(0001), ZnO(0001̅ ), and ZnO(101̅ 4) single crystal surfaces by scanning tunneling microscopy, high-energy grazing incidence X-ray diffraction, low-energy electron diffraction, and scanning electron microscopy. The (111) oriented Cu NPs on basal ZnO showed only small area fractions of high indexed Cu(225) and Cu(331) facets. Cu NPs grown on ZnO(101̅ 4) show alignment of Cu [111] with the ZnO [0001] direction, which is at an angle of 24.8°to the ZnO(101̅ 4) surface normal. Because of this tilt, the NPs exhibit a shape with a larger fraction of high indexed facets such as ( 335), ( 221), ( 113), and (551̅ ). In addition, the direct interaction of subsequent Cu(111) planes to the underlying substrate results in unequal amounts of ABCA and ACBA stacked NPs. Small NPs are found to interact strongly with the vicinal surface, giving rise to a surface corrugation with a multiple of the surface step distance. The high density of lowcoordinated Cu surface atoms potentially increases the overall catalytic activity for methanol synthesis and CO 2 hydrogenation reactions.
We report on differences in the magnetite (111) surface structure when treated in two different ways. Both preparations are done under UHV conditions at elevated temperatures, but in one case the sample is cooled down while keeping it in an oxygen atmosphere. Scanning tunneling microscopy after each of the preparations shows a different apparent morphology, which is discussed to be an electronic effect and which is reflected in the necessity of using opposite bias tunneling voltages in order to obtain good images. Surface x-ray diffraction reveals that both preparations lead to Fe vacancies, leading to local O-terminations, the relative fraction of which depends on the preparation. The preparation under reducing conditions leads to a larger fraction of Fe-termination. The geometric structure of the two different terminations is found to be identical for both treatments, even though the surface and near-surface regions have small compositional changes; after the oxidizing treatment they are iron deficient. Further evidence for the dependence of iron vs. oxygen fractional surface terminations on preparation comes from Fourier transform infrared reflection-absorption spectroscopy, which is used to study the adsorption of formic acid. These molecules dissociate and adsorb in chelating and bidentate bridging geometries on the Fe-terminated areas and the signal of typical infrared absorption bands is stronger after the preparation under reducing conditions, which results in a higher fraction of Fe-termination. The adsorption of formic acid induces an atomic roughening of oxidized magnetite (111) surface which we conclude from the quantitative analysis of the crystal truncation rod data. The roughening process is initiated by atomic hydrogen, which results from the dissociation of formic acid after its adsorption on the surface. Atomic hydrogen adsorbs at surface oxygen and after recombination with another H this surface hydroxyl can form H2O, which desorbs from the surface, while iron ions diffuse into interstitial sites in the bulk.
Water in confinement exhibits altered properties in molecular arrangement, bonding and interaction with its neighboring environment, as compared to its bulk counterpart. In this work, periodically arranged D 2 O nano droplets of ∼1 nm size on top of a graphene-iridium moiré superstructure were investigated by Fourier transformation reflection absorption infrared spectroscopy under ultrahigh vacuum conditions at ∼120 K. The infrared bands of D 2 O clusters differ significantly from those observed for bulk D 2 O amorphous solid water or crystalline ice phases. Blue shifted symmetric and asymmetric stretching bands with narrower band widths and modified band intensity ratios were observed, pointing to an enhanced internal order and a reduced nearest neighbor distance. Furthermore, two IR bands of 'dangling' deuterium atoms were detected originating from threefold coordinated water molecules at the surface of the clusters and at their interface to the graphene layer. The latter arose only with the transition from the water clusters to an amorphous solid water layer. We propose that upon coalescence, opposing local dipoles trigger a hydrogen bond rearrangement 1
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